The present invention relates to a circuit breaker of so-called spiral arc type, in which the current is shut off by forming the arc in the shape of spiral, for use, for example, in a high voltage d.c. circuit and, particularly, to a circuit breaker having an improved arc extinction chamber.
D.C. circuit breakers fall into the puffer blast type and the spiral arc type. The puffer blast type disadvantageously needs a large operating force for blasting the arc. Whereas, the spiral arc type, in which the magnetic field is applied to the tips of contacts in the arc extinction medium (e.g., SF.sub.6 gas) so as to stretch the arc in the shape of spiral thereby to increase the arc voltage as high as the power voltage so that a high-voltage, large current is shut off, can produce a high arc voltage between less distant electrodes due to the sprially shaped arc, and needs a small operating force merely for driving the contact electrodes, allowing advantageously a compact and light weight design.
FIG. 1 is a sectional view showing, as an example, the principal portions of the arc extinction chamber of the conventional d.c. circuit breaker of spiral arc type. The arrangement includes a cylindrical fixed contact 1, a movable contact 2 formed in the shape of deformed cylinder with the E-shaped cross section and disposed detachably and coaxially with respect to the fixed contact 1, an excitation winding 3 disposed coaxially outside both contacts 1 and 2 and adapted to produce the magnetic field H in parallel to the central axis of the contacts 1 and 2, and an insulator 4 made of Teflon and the like attached to the surface of the movable contact 2 confronting the fixed contact 1.
The operation of the above-mentioned conventional spiral-arc d.c. circuit breaker will be explained. When arc 5 is created by the separation of the contacts 1 and 2, the current flowing in the portion referred to by 6 in the movable contact 2 is opposite in direction to the current caused by the arc 5, resulting in the generation of an electromagnetic reaction force between both currents, and the arc 5 is pushed outwardly as shown. This creates orthogonal components in the current caused by the arc 5 and in the magnetic field H, producing an electromagnetic force based on the Fleming's left-hand rule between the components, and both ends of the arc 5 move oppositely along the circumferential direction and the arc grows into a spiral. As a result, the arc voltage increases, providing the ability of shutting off a high-voltage, large d.c. current.
On this account, it is a prerequisite for the conventional spiral-arc circuit breaker to reverse the direction of the current flowing in the movable contact 2, resulting disadvantageously in a complex structure of the movable contact 2. In addition, generation of a magnetic field having a component parallel to the moving direction of the movable contact in the vicinity of the contact section causes the aerial magnetic path to become longer as the movable contact has a longer stroke, resulting disadvantageously in a large magnetomotive force needed for the excitation winding.